Software-defined networking (SDN) is an umbrella term encompassing several kinds of network technology aimed at making the network as agile and flexible as the virtualized server and storage infrastructure of the modern data center. The goal of SDN is to allow network engineers and administrators to respond quickly to changing business requirements. In a software-defined network, a network administrator can shape traffic from a centralized control console without having to touch individual switches, and can deliver services to wherever they are needed in the network, without regard to what specific devices a server or other hardware components are connected to. The key technologies for SDN implementation are functional separation, network virtualization and automation through programmability.
Originally, SDN focused solely on separation of the control plane of the network, which makes decisions about how packets should flow through the network from the data plane of the network, which actually moves packets from place to place. When a packet arrives at a switch in the network, rules built into the switch's proprietary firmware tell the switch where to forward the packet. The switch sends every packet going to the same destination along the same path, and treats all the packets the exact same way. In a classic SDN scenario, rules for packet handling are sent to the switch from a controller, an application running on a server somewhere, and switches (also known as data plane devices) query the controller for guidance as needed, and provide it with information about traffic they are handling. Controllers and switches communicate through a controller's south bound interface, usually OpenFlow, although other protocols exist.
Where a traditional network would use a specialized appliance such as a firewall or link-load balancer, an SDN deploys an application that uses the controller to manage data plane behavior. Applications talk to the controller though its north-bound interface. As of the end of 2014, there is no formal standard for the application interface of the controller to match OpenFlow as a general south-bound interface. It is likely that the OpenDaylight controller's northbound application program interface (API) may emerge as a defacto standard over time, given its broad vendor support.
Software-defined networking uses an operation mode that is sometimes called adaptive or dynamic, in which a switch issues a route request to a controller for a packet that does not have a specific route. This process is separate from adaptive routing, which issues route requests through routers and algorithms based on the network topology, not through a controller.
With SDN, the administrator can change any network switch's rules when necessary -- prioritizing, de-prioritizing or even blocking specific types of packets with a very granular level of control. This is especially helpful in a cloud computing multi-tenant architecture, because it allows the administrator to manage traffic loads in a flexible and more efficient manner. Essentially, this allows the administrator to use less expensive commodity switches and have more control over network traffic flow than ever before.
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